Method and apparatus for mixing bituminous mixtures

ABSTRACT

Granular mineral components and bituminous binder components are mixed in a rotating drum by means of longitudinally extending chains which rotate along with the drum and which repeatedly lift the materials to be mixed and drop these materials again downstream of the preceding lifting point and which chains on their returning leg in each revolution provide a self cleaning action by contacting the inner surface of the drum. The apparatus for performing this method comprises a plurality of chains extending longitudinally within the drum and connected thereto by means which permit an adjustment of the slack in the chains. Preferably, the links of the chains are plates which are hinged to each other and which are adjustable in their angular position relative to a radial plane of the drum.

United States Patent [191 Muntzer et al.

[ 1 ,iur1e5,1973

1541 METHOD AND APPARATUS FOR MIXING BITUMINOUS MIXTURES [75] Inventors: Emile Muntzer; Paul Muntzer, both of Strasbourg, France 22 Filed: Nov. 23, 1971 21 Appl. No.: 201,494

[73] Assignee:

[30] Foreign Application Priority Data Dec. 2, 1970 France 1.7043989 [52] U.S. Cl. 259/146, 259/81, 259/175 [51] Int. C1. ..B01f 9/04 [58] Field of Search ..259/148, 177,3, 259/176, 175, 81, 89; 90, 14, 15, 30, 31,178 R, 146

[56] References Cited UNITED STATES PATENTS 2,261,403 11/1941 Newhouse ..259/89 2.138.240 11/1938 Kobusch .259/3 288,919 11/1883 llrccr 4 1 259/89 738,800 9/1903 Hocking ..259/89 Primary ExaminerRobert W. Jenkins Att0rneyW. G. Fasse [57] ABSTRACT Granular mineral components and bituminous binder components are mixed in a rotating drum by means of longitudinally extending chains which rotate along with the drum and which repeatedly lift the materials to be mixed and drop these materials again downstream of the preceding lifting point and which chains on their returning leg in each revolution provide a self cleaning action by contacting the inner surface of the drum. The apparatus for performing this method comprises a plurality of chains extending longitudinally within the drum and connected thereto by means which permit an adjustment of the slack in the chains. Preferably, the links of the chains are plates which are hinged to each other and which are adjustablein their angular position relative to a radial plane of the drum.

19 Claims, 8 Drawing Figures SHEET 10F 2 PATENTEDJUH 5 I975 FIG. 3

PATENTEDJUH 5197a 3 737 149 SHEET 20F 2 FIG.6

METHOD AND APPARATUS FOR MIXING BITUMINOUS MIXTURES BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for mixing bituminous mixtures. More specifically, the present method and apparatus facilitate the thorough intermixing of the starting components, whereby a rotating drum especially a so called drum drier is used for performing the present method. The drum has an inlet opening and an outlet opening and is arranged with an inclination relative to the horizontal. The starting materials including moist mineral components and bituminous binder components, especially liquid hot bituminous components are supplied to said inlet and are advancing through the drum toward the outlet. During this advance, the components are thoroughly intermixed in accordance with the present teachings.

The above mixing combines moist and hot mixing procedures since the mineral components are, as mentioned, moist or wet whereas the bituminous components are hot. This combined mixing has found practical acceptance as the so called dustless mixing because initially the dust particles are kept bound to the coarse granules by the water or moisture and thereafter the bituminous binder components retain the dust particles.

Several methods are known for performing the so called dustless preparation of bituminous mixtures. For example, a drum drier which is heated in a countercurrent manner may be used for the heating, drying and for continuously converting the components or even the premixed components into a thoroughly intermixed mixture. Generally, these drum driers are referred to as the activator regardless whether the drum drier is arranged for the above mentioned countercurrent heating or for heating whereby the heat flows in the same direction as the direction of travel of the mixture through the activator.

In activators which operate in accordance with the so called countercurrent method, that is, the heat flows in a direction contrary to the advance of the mixture, the latter is not lifted any more in the portion of the activator adjacent to its outlet. Such portion serves simultaneously as the combustion chamber for the heating device, that is, the flame or rather the heating gases are burning in this portion of the drum and such portion takes up about one quarter of the entire length of the drum and there are no means provided inside the drum in this portion for lifting or raising the mixture which advances toward the outlet by sliding along the drum bottom due to the inclination of the drum and due to its rotation.

Such sliding or gliding transport of the mixture through the exit portion of the drum is necessary on the one hand in order to avoid over heating of the mixture and thus damaging the bituminous binder components, which, during this end phase of the passage through the activator are nevertheless heated to temperatures quite close to the permissable temperature limit. n the other hand such sliding transport does not improve the quality of the mixture, rather it might even impair the quality of the mixture.

Due to the high temperatures in the end portion of the drum operating under the countercurrent method and due to the relatively limited movement of the mixture flow, this portion of the drum is especially subject to incrusting the bake-on of the mixture. Attempts have been previously made to overcome these bake-on and incrusting problems. Thus, chains have been arranged heretofore in the exit end of the drum whereby especially link or long link chains such as ships chains or the like have been used and whereby merely one end of the chains was attached to the inner wall of the drum. These chains have a length which in any event corresponds to the diameter of the drum and which mayeven be longer than said drum diameter.

Where the drum drier or rather activator is operated in the so called direct current method that is the heat flow and the mixture advance in the same direction, the inner walls of the drum and the internal structure thereof normally remain free of baked-on mixture portions and deposits. This is due to the fact that the direct current method is more suitable for the dustless preparation of the mixtures since the temperature of the mixture or of the premixed components is limited by the evaporation heat which is required for the removal of the water or moisture content. As a result, the mixture reaches the necessary temperature only after its continuous advance through the drum, that is in drum regions into which the flame does not reach. Accordingly, it is possible in connection with the so called direct current method to move the mixture, that is, to lift and drop it all the way into the exit area of the drum.

However, this direct current method is still subject to substantial difficulties in that the transporting, lifting and throwing means inside the drum drier produce a large number of so called dead corners, especially along their sides facing the internal drum wall. These dead corners facilitate the accumulation of particles, especially of small particles and the bake-on of these particles, whereby even larger particles are retained in these comers.

OBJECTS OF THE INVENTION In view of the foregoing, it is the aim of the invention to achieve the following objects singly or in combination:

to remove the above outlined drawbacks of the prior art, more specifically, to transport the mixture through the drum drier while simultaneously avoiding the bakeon and incrusting of the mixture;

to provide a mixing and drying method in which the mixture is repeatedly lifted and dropped as it travels through the length of the drum;

to provide a method in which the mixture is continuously and repeatedly subjected to kneading action, followed by a free falling phase, followed by a kneading action, followed by a free falling phase and so forth;

to increase the intermixing or rather to improve the mixing intensity;

to provide a so called drum drier in which internal means such as chains having an adjustable slack repeatedly lift and drop the mixture while simultaneously advancing the same and also providing a cleaning action inside the drum to avoid such bake-on and incrusting; and

to provide a so called drum drier in which the above mentioned dead corners are avoided.

SUMMARY OF THE INVENTION The above objects have been achieved according to the invention in a method which employs a drum drier whereby the components to be mixed or the premixed components which are introduced at an inlet end of the drum are subjected to the action of a chain system arranged substantially coaxially within the drum. The chain system comprises preferably a plurality of chains which are evenly distributed over the inner circumference of the drum. During each revolution each chain of the system passes through a lifting or ascending phase which is followed by a dropping phase, wich in turn is followed by a descending or return phase. During the ascending phase the chains lift the mixture whereby the mixture is subjected to a mechanical working or kneading action between the respective chain and the inner surface of the drum wall. As any particular chain continues its ascending phase and into the dropping phase, the mixture is transported in the direction toward the outlet due to the inclination of the drum and due to the slack in the chains. Thus, such lifting and dropping actions follow each other in an overlapping fashion and in timed relationship relative to each other from the drum input toward the drum output. Further, as the chains begin their descend, they fall against the inner wall of the drum whereby a cleaning action is accomplished since the chains now contact the drum while the drum is not in contact with the mixture in this portion of its rotation. The contact between the chains and the drum begins intermediate the ends of any particular chain and procedes from there toward the ends of the chain.

According to a further feature of the invention, the present method provides for accelerating or decelerating the flow of the mixture by providing chains which links differing in their shape and/or effective surface and arranged in sequential order. Rather than varying the effective area in steps from link to link, it is also possible to vary the effective area in a continuous fashion.

The apparatus according to the invention comprises a drum in the inside of which there are arranged chains secured to the inner drum wall at both ends, preferably in such a manner that the chains extend coaxially to the longitudinal axis of the drum and at uniform peripheral spacings from each other. The chains may substantially correspond to the length of the drum or they may be shorter and preferably they are arranged in a radial fashion. The chain links are preferably in the form of plates hinged to each other, whereby, as mentioned, the plates are arranged to extend radially inside the drum. Although plate chain links have been found to be especially effective, it is also possible to employ links of different shape whereby such shapes or effective surface area of the chain links will be selected in accordance with the nature of the components to be mixed. The individual chain links may have the same shape and/or size or they may have differing shapes and or sizes along the length of each individual chain. In this connection the invention teaches that the shape and size of the chain links will depend upon the size of the granular material. Thus, vigorous throwing motions would norm ally be avoided in connection with the mixing of predominantly finely grained briquetting material and this will be accomplished by using relatively small round chain links in order to produce a rather revolving motion of the material or mixture. Contrary thereto, where a throwing motion is desired, for example, in connection with coarse materials, plate chain links will preferably be employed.

The invention further teaches to attach the chain ends adjacent to the outlet end of a drum to adjustable means whereby the slack of the chains may be adjusted as well as the spacing between the chains and the inner surface of the drum walls and also the angular position of the chain links relative to a radial plane.

The just mentioned adjustability of the chain position inside the drum provides the advantage that the motion of the mixture may be controlled whereby a throwing motion may be varied into a more gentle rolling or re volving 'motion. Besides, the arrangement of the adjusting means at the outer end of the drum has the advantage that these means do not interfere with the desired and uninterrupted discharge of the mixture.

BRIEF FIGURE DESCRIPTION In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 is a perspective view into a drum drier according to the invention, whereby part of the wall of the drum has been removed in order to expose the arrangement of the chains;

FIG. 2 is a sectional view through the drum drier according to FIG. 1, whereby the section extends perpendicularly to the longitudinal axis of the drum drier;

FIG. 3 illustrates in a schematic fashion the variation of the shape and size of the chain links in a step fashion;

FIG. 4 illustrates a view, partially in section, in the direction toward the outlet end of the drum to show the means for adjusting the angular position of the chains;

FIG. 5 is a view also partially in section along'the line VV in FIG. 4;

FIG. 6 is a sectional view through the drum to illus trate the attachment of the chains at the upstream end thereof;

FIG. 7 illustrates an example of how adjacent chain links may be connected to each other; and

FIG. 8 illustrates another embodiment wherein the shape and size of the chain links vary continuously from one end to the other.

DESCRIPTION OF EXAMPLE EMBODIMENTS Referring to F IG. 1, the drum drier 1 according to the invention has an upstream inlet end 2 and a downstream discharge end of outlet 3. The drum 1 is supported at an inclination'relative to the horizontal so that its inlet end 2 will be at a higher elevation than its outlet end 3. Further, the drum 1 is supported for rotation in the direction of the arrow A.

The mixture 4 is introduced at the inlet end of the drum and travels through the drum in the direction of the arrow B.

According to the invention, there is provided a system of longitudinally extending chains 5 to 12 as best seen in FIGS. 1 and 2.

Each chain comprises a plurality of chain links 13 which may have different sizes and shapes as will be described in more detail below. Further, each chain is adjustable in a radial direction relative to a radial plane as indicated by the arrow C (FIG. 5). The chains are also adjustable by an angle a in either direction relative to said radial plane. The number of chains in a system may vary depending on the particular requirements.

As the drum 1 rotates in the direction of the arrow A it passes, and with it the chains, during each revolution through the quadrants I, II, III, and IV as shown in FIG. 2. As the drum passes through the quadrants I and II the chains are on an ascending phase of their travel. In the quadrants III and IV the chains are in a descent phase of their travel. In the quadrant I the chains lift the mixture 4. In the quadrant II and possibility also partially in the quadrant III, the chains drop the mixture 4 and in the quadrant III and IV the chains perform a cleaning action as will also be described in more detail below.

For simplicitys sake, the entire length of the drum drier is not shown. Thus, ahead of the inlet end, there is arranged the inlet proper which simultaneously forms the flame zone. Preferably, but not necessarily limited thereto, the flame zone is free of any internal structure in order to avoid the throwing of the mixture components through the hot gas zone, thus also avoiding damage to the binder components.

In the embodiment shown in FIGS. 1 and 2, the chains 5 to 12 comprise the chain links 13 in the form of plates hinged to each other and preferably made of heavy temperature resistant ingot steel. Referring to FIG. 6, the link 13 adjacent to the inlet end 2 is secured to the inner wall of the drum 1 by a hinge 14 connected to a stud 15, which in turn is secured to the drum wall, for example, by welding. A further hinge 16 interconnects adjacent links 13. A different possibility for interconnecting adjacent links 13 is shown in FIG. 7 where side straps l7 hinge adjacent links to each other.

The stud will preferably point radially inwardly, whereas as shown in FIGS. 4 and 5, the means for securing the chain ends adjacent to the drum outlet are attached to the outer peripheral surface of the drum, thus forming an extension thereof and providing the advantage that the exit of the mixture from the drum is not impeded. Moreover, this arrangement outside of the drum facilitates the adjustment of the chain position as will be described.

Referring again to FIG. 2, there are shown auxiliary chains 18 which interconnect adjacent longitudinal chains 5 to 12 and which act as safety chains. A plurality of such safety chains may be arranged at intervals along the longitudinally extending chains. These safety chains prevent substantial damage if any one of the longitudinal chains should break because in such an instance, only the portions of the longitudinal chains intermediate to auxiliary chains may become free to hang into the inner space of the drum.

FIG. 3 illustrates an embodiment of a chain comprising a plurality of links 13a to 13d having different sizes and/0r shapes, whereby preferably, the larger size links 13a would be arranged adjacent to the inlet end while the size of the other links decreases in steps towards the outlet end. The embodiment of FIG. 8 illustrates the continuous decrease of the size of the chain links also preferably from the inlet end toward the outlet end.

Another possibility of changing the effective surface area of the chains is provided by adjusting the angle a between the radial plane and the respective chain link as shown in FIG. 2. Such angular adjustment may be accomplished as shown, for example, in FIGS. 4 and 5. For this purpose the means for securing the chain ends to the outlet end of the drum comprise a first member 19 connected to the outer peripheral end of the drum by means of a hinge 20. The angular position of the member 19 is adjustable by adjusting screws 21 and 22 which when tightened will hold the member 19 in the desired position. The member 19 comprises a radially inwardly extending leg 23 to which the last chain link 13 is secured by a slider block 24 and perferably also through tensioning means such as a turn buckle 25. The spacing 26 between the chain and the inner surface of the drum may be adjusted by sliding the block 24 up and down along the leg 23. The block may be fixed in an adjusted position by a set screw 27. Furthermore, the turn buckle 25 permits adjusting the tension in the chain and thus the slack.

Incidentally, the size of the slack will be determined by the desired intensity of intermixing and also by the economically justifiable wear and tear. The spacing between adjacent auxiliary chains 18 will also be determined by the permissable wear and tear. Preferably, theses spacings should be such that only relatively short lengths of longitudinal chain portions should'be permitted to hunt into the inner space of the drum when a longitudinal chain should break.

Preferably the means for securing any one of the chains at both ends thereof should be aligned with each other in the longitudinal direction. However, it is also possible to displace the securing means relative to the longitudinal direction whereby the chains will be skewed.

In operation, the chains will continuously and sequentially lift the material 4 at positions upstream relative to positions where the material is dropped again as the chains move through the second quadrant (FIG.2). Further, in the first quadrant, the mixture components will be subjected to a kneading action between the chain and the inner wall of the drum. When the material is dropped a free falling effect is accomplished whereby not only a more efficient mixing and thus a better mixed product is accomplished but also the above mentioned bake-on and the incrusting are avoided with certainty. This has the further advantage that large agglomerated pieces do not occur and thus cannot interfere with the proper operation of the chain system inside the drum. Another advantage of the invention is seen in the above described adjustability of the position, and/or size, and/or effective area of the individual chain links whereby the travel of the mixture through the drum may be accelerated or decelerated as desired without changing the inclination of the drum relative to the horizontal. This feature constitutes a substantial advance in the art since it assures the most efficient use of the heat employed while simultaneously avoiding damage to the binder components and preventing said bake-on and incrusting.

Although the invention has been described with reference to specific example embodiments, it is to be understood that it is intended to cover all modifications and equivalents within the scope of the appended claims.

What is claimed is:

1. In a method for mixing bituminous mixtures in a drum drier rotatably supported with an inclination relative to the horizontal whereby a drum inlet is positioned higher than a drum outlet, said mixtures including, as starting materials, moist mineral components and bituminous binder components, especially liquid hot bitumen, the improvement comprising rotating together with said drum a plurality of chains extending longitudinally but with sag inside said drum intermediate said drum inlet and said drum outlet, whereby said chains move repeatedly through an ascending portion, through a discharge portion, and through a descending portion during each revolution, repeatedly lifting said materials by said chains while the chaims move through said ascending portion of their travel, whereby the materials are subjected to mechanical working between the chain and the inner surface of the drum, repeatedly dropping said materials from the chains as the chains vary their positions relative to the vertical while moving through said discharge portion of their travel whereby due to said drum inclination of chain sag, a continuous overlap of dropping materials is created in time and space relationship from said drum inlet to said drum outlet for mixing and simultaneously transporting said materials through the drum, and sequentially contacting said chains and said drum during said descending portion of chain travel whereby a cleaning action is provided for avoiding material accumulations.

2. The method according to claim 1, wherein said cleaning action cleans the chains as well as the drum wall.

3. The method according to claim 1, further comprising accelerating the passage of said materials from said drum inlet to said drum outlet.

4. The method according to claim 1, further comprising decelerating the passage of said materials from said drum inlet to said drum outlet.

5. In an apparatus for mixing bituminous mixtures, as by means of a rotatable drum drier having an inlet at one end thereof and an outlet at the opposite end, said drum being located with an inclination relative to the horizontal, the improvement comprising a plurality of chains, a like plurality of first means for securing one end of each of said chains to the inside of said drum intermediate its ends, a like plurality of second means longitudinally spaced from said first means and positioned relative to the drum for securing the other end of each of said chains, whereby said chains extend with sags longitudinally inside said drum substantially adjacent to the inner walls thereof, and intermediate the ends of the drum, said first securing means being connected to the inner surface of the drum in a common radial plane located adjacent to said inlet end of the drum, and said apparatus further comprising means for connecting said securing means to the drum adjacent to said outlet end.

6. The apparatus according to claim 5, wherein said first and second securing means are longitudinally aligned in pairs whereby said chains extend in parallel to each other and coaxially relative to said drum.

7. The apparatus according to claim 5, wherein said first and second securing means are evenly spaced relative to the inner circumference of said drum.

8. The apparatus according to claim 5, wherein said chains comprise links, all of which have the same shape.

9. The apparatus according to claim 5, wherein said chains comprise links of differing shapes.

10. The apparatus according to claim 5, wherein said chains comprise links, all of which have the same size.

11. The apparatus according to claim 5, wherein said chains comprise links having differing sizes.

12. The apparatus according to claim 5, wherein said chains comprise plate links and hinge means for interconnecting said plate links.

13. The apparatus according to claim 5, wherein said second securing means are arranged adjacent said outlet end outside the drum proper, each of said second securing means comprising a first member, means for hinging said first member to the drum at said outlet end thereof, whereby said first member extends longitudi' nally of said drum, means for adjusting the angular position of said first member relative to the drum, a second member connected with one end to said plate member and extending with its free end radially inwardly relative to the drum, and means for adjustably connecting an end of the respective chain to said free end of the second member whereby the spacing between the chain and the inner drum wall is adjustable.

14. The apparatus according to claim 13, wherein said first member is hinged to the outer surface of the drum and has a portion which extends over the outlet end of the drum, said one end of the second member being connected to said portion of the first member whereby the angular position of the second member relative to a radial direction is adjustable by said adjustable means which secure the first member in different hinged positions.

15. The apparatus according to claim 5, wherein said second securing means comprise tensioning means for controlling the sag of each chain.

16. The apparatus according to claim 5, further comprising auxiliary chain means interconnecting said first mentioned chains and extending circumferentially from chain to chain inside said drum.

17. The apparatus according to claim 16, wherein a plurality of said auxiliary chain means are distributed along the length of the drum and connected to said first mentioned chains.

18. The apparatus according to claim 5, wherein said chains comprise links the effective surface of which varies in steps from said inlet toward said outlet.

19. The apparatus according to claim 5, wherein said chains comprise links the effective surface of which varies continuously from said inlet toward said outlet. t

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent "R0. 3 Dated June 9 9 5 lnvento- Emile Muntzer et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

1 Claim 1, column 7, line 9 "of" should read m and a Claim 5, column 7 line 26, delete "as"o Claim 5, column 7, line A3, after "said" insert second Claim 13, column 8.; line l after "outside" insert of a Claim i l, column 8, lines 52 and 33 "adjustable" should read adjusting Signed and sealed this 19th day of November 1974.

(SEAL) Attest:

5 MCCOY M. GIBSON JR. v c. MARSHALL DANN Attestin g officer f 4 Commissioner of Patents FORM PO-IOSO (10-69) USCOMM-DC 603764 69 LLS. GOVERNMENT PRINTING OFFICE I909 OS66-33l, 

2. The method according to claim 1, wherein said cleaning action cleans the chains as well as the drum wall.
 3. The method according to claim 1, further comprising accelerating the passage of said materials from said drum inlet to said drum outlet.
 4. The method according to claim 1, further comprising decelerating the passage of said materials from said drum inlet to said drum outlet.
 5. In an apparatus for mixing bituminous mixtures, as by means of a rotatable drum drier having an inlet at one end thereof and an outlet at the opposite end, said drum being located with an inclination relative to the horizontal, the improvement comprising a plurality of chains, a like plurality of first means for securing one end of each of said chains to the inside of said drum intermediate its ends, a like plurality of second means longitudinally spaced from said first means and positioned relative to the drum for securing the other end of each of said chains, whereby said chains extend with sags longitudinally inside said drum substantially adjacent to the inner walls thereof, and intermediate the ends of the drum, said first securing means being connected to the inner surface of the drum in a common radial plane located adjacent to said inlet end of the drum, and said apparatus further comprising means for connecting said securing means to the drum adjacent to said outlet end.
 6. The apparatus according to claim 5, Wherein said first and second securing means are longitudinally aligned in pairs whereby said chains extend in parallel to each other and coaxially relative to said drum.
 7. The apparatus according to claim 5, wherein said first and second securing means are evenly spaced relative to the inner circumference of said drum.
 8. The apparatus according to claim 5, wherein said chains comprise links, all of which have the same shape.
 9. The apparatus according to claim 5, wherein said chains comprise links of differing shapes.
 10. The apparatus according to claim 5, wherein said chains comprise links, all of which have the same size.
 11. The apparatus according to claim 5, wherein said chains comprise links having differing sizes.
 12. The apparatus according to claim 5, wherein said chains comprise plate links and hinge means for interconnecting said plate links.
 13. The apparatus according to claim 5, wherein said second securing means are arranged adjacent said outlet end outside the drum proper, each of said second securing means comprising a first member, means for hinging said first member to the drum at said outlet end thereof, whereby said first member extends longitudinally of said drum, means for adjusting the angular position of said first member relative to the drum, a second member connected with one end to said plate member and extending with its free end radially inwardly relative to the drum, and means for adjustably connecting an end of the respective chain to said free end of the second member whereby the spacing between the chain and the inner drum wall is adjustable.
 14. The apparatus according to claim 13, wherein said first member is hinged to the outer surface of the drum and has a portion which extends over the outlet end of the drum, said one end of the second member being connected to said portion of the first member whereby the angular position of the second member relative to a radial direction is adjustable by said adjustable means which secure the first member in different hinged positions.
 15. The apparatus according to claim 5, wherein said second securing means comprise tensioning means for controlling the sag of each chain.
 16. The apparatus according to claim 5, further comprising auxiliary chain means interconnecting said first mentioned chains and extending circumferentially from chain to chain inside said drum.
 17. The apparatus according to claim 16, wherein a plurality of said auxiliary chain means are distributed along the length of the drum and connected to said first mentioned chains.
 18. The apparatus according to claim 5, wherein said chains comprise links the effective surface of which varies in steps from said inlet toward said outlet.
 19. The apparatus according to claim 5, wherein said chains comprise links the effective surface of which varies continuously from said inlet toward said outlet. 